Conventional methods for obtaining fresh water from seawater include an evaporation method in which seawater is evaporated, and a reverse osmosis method in which fresh water is obtained by filtering seawater through a filtration membrane called a reverse osmosis membrane (RO membrane) by applying pressure to the seawater, concentrating the salt in the seawater, and discarding thereof.
The latter reverse osmosis method is energy efficient compared to the evaporation method. However, in the reverse osmosis method, a careful pretreatment (treatment with an “ultrafilter membrane (UF membrane)” or a “microfilter membrane (MF membrane)” for removing suspended matters in seawater, or raw water) is required to prevent the RO membrane from being clogged by microbes and deposits in seawater, and the maintenance and the like is expensive.
It is also difficult to obtain water quality as good as that produced by the evaporation method. To obtain highly purified water quality, a plurality of stages of reverse osmosis devices needs to be combined.
FIG. 10 is a schematic of a multi-stage seawater desalination apparatus of a conventional reverse osmosis method.
As shown in FIG. 10, a conventional multi-stage seawater desalination apparatus 100 includes a high-pressure pump P1 that increases the pressure of raw water (seawater) 101 from which suspended matters are removed by pretreatment to a predetermined high pressure, a high-pressure reverse osmosis device 103 that includes a high-pressure reverse osmosis membrane 103a for concentrating a salt content in high-pressure feed water 102 whose pressure is increased by the high-pressure pump P1, an intermediate tank 110 that temporarily stores therein permeated water 104 that has passed through the high-pressure reverse osmosis device 103, a low-pressure pump P2 that reduces the pressure of the permeated water 104 supplied from the intermediate tank 110 to a predetermined low pressure, and a low-pressure reverse osmosis device 106 that has a low-pressure reverse osmosis membrane 106a for concentrating a salt content in low-pressure feed water 105 whose pressure is reduced by the low-pressure pump P2, and obtains product water (fresh water) 107. Because the pressure in the intermediate tank 110 is returned to normal, the pressure is released. While the system is stopped, pH of the permeated water 104 is adjusted, by adding a pH adjusting agent 111, thereby preventing microbial contamination (see Non-Patent Document 1 “Fukuoka District Waterworks Agency: Mechanism of Seawater Desalination”).
FIG. 10 is a schematic of concentrated water 103b from the high-pressure reverse osmosis device 103, and concentrated water 106b from the low-pressure reverse osmosis device 106.
[Non-Patent Document 1] “Fukuoka District Waterworks Agency: Mechanism of Seawater Desalination” URL:http://www.f-suiki.or.jp/seawater/facilities/mechanism.php